The present invention relates to an optical fiber for amplification and an optical fiber amplifier using the same for use in optical communications, which are suitable for the case of reducing the difference in gains between an LP01 mode light beam and an LP11 mode light beam.
In optical communications, signals are superposed on light beams propagating through an optical fiber for communications. In order to increase information volumes transmitted through a single core in such optical communications, there are known few-mode communications in which information is superposed on an LP01 mode (a fundamental mode) light beam and on an LP11 mode light beam of signal light beams for information communications. Therefore, in the case of amplifying signal light beams propagating through an optical fiber using an optical fiber amplifier, it is necessary to amplify both of the LP01 mode and LP02 mode light beams.
Non Patent Literature 1 below describes an optical fiber for amplification in which the core of the optical fiber for amplification has a step refractive index, i.e., the refractive index of the core is constant in the radial direction. The core is doped with erbium. The concentration of erbium is constant in the core. However, in the case of amplifying light beams using such an optical fiber for amplification, the gain of the LP01 mode light beam is often different from the gain of the LP11 mode light beam.
Therefore, Non Patent Literature 2 below describes an optical fiber for amplification aiming for reducing such gain differences. The optical fiber for amplification described in Non Patent Literature 2 has the refractive index profile of the core similar to the refractive index profile of the core of the optical fiber for amplification described in Non Patent Literature 1. However, in the optical fiber for amplification in Non Patent Literature 2, erbium is not doped in the center region including the center axis of the core, and erbium is doped in the outer region surrounding the center region. Consequently, the power of the LP01 mode light beam and the power of the LP11 mode light beam propagating through the region doped with erbium are adjusted, and thus the gain of the LP01 mode light beam is made equal to the gain of the LP11 mode light beam.
[Non Patent Literature 1] Y. Yung et al., “ First demonstration of multimode amplifier for spatial division multiplexed transmission systems,” Proc. ECOC '11, Th.13.K4 (2011)
[Non Patent Literature 2] G. Le Cocq et al., “Modeling and characterization of few-mode EDFA supporting four mode groups for mode division multiplexing,” Opt. Express 20, 27051-27061 (2012)